Rotating disc optical synchronization system using binary diffractive optical elements

ABSTRACT

An optical synchronization system uses a stationary binary diffractive optical element and a rotating disc with alternating binary diffractive optical elements and blocking elements to create a moire pattern of a synchronization signal for a pixel clock.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application contains subject matter that is related to subjectmatter of patent application Ser. No. 07/989,321, filed Dec. 11, 1992,and patent application Ser. No. 07/989,313, filed Dec. 11, 1992, bothassigned to the same assignee herein.

BACKGROUND OF THE INVENTION

This invention relates to an optical synchronization system for arotating disc, and, more particularly, to an optical synchronizationsystem with binary diffractive optical elements for a rotating disc.

The propagation of a light beam can be changed by three basic means:reflection by a mirror, refraction by a lens and diffraction by agrating. Optical systems traditionally rely on reflection and refractionto achieve the desired optical transformation. Optical design, based onmirror and lens elements, is a well-established and refined process.Until recently, the problems with diffraction and fabricating highefficiency diffractive elements have made diffractive elementsunfeasible components of optical systems.

The diffractive process does not simply redirect a light beam.Diffraction, unlike refraction and reflection, splits a light beam intomany beams--each of which is redirected at a different angle or order.The percentage of the incident light redirected by the desired angle isreferred to as the diffraction efficiency. The diffraction efficiency ofa diffractive element is determined by the element's surface profile. Ifthe light that is not redirected by the desired angle is substantial,the result will be an intolerable amount of scatter in the image oroutput plane of the optical system.

Theoretically, diffractive phase elements can achieve 100 percentdiffraction efficiency at a given wavelength. To achieve thisefficiency, however, a continuous phase profile is necessary. Thetheoretical diffraction efficiency of this surface profile is alsorelatively sensitive to a change in wavelength. By contrast, refractiveelements are wavelength insensitive. The technology for producing highquality, high efficiency, continuous phase profiles does not presentlyexist.

A compromise that results in a relatively high diffraction efficiencyand ease of fabrication is a multi-level phase grating. The larger thenumber of discrete phase levels, the better the approximation of thecontinuous phase function. These multi-level phase profiles can befabricated using standard semiconductor integrated circuit fabricationtechniques.

As disclosed in Binary Optics Technology: The Theory and Design ofMulti-level Diffractive Optical Elements by G. J. Swanson of the LincolnLaboratory at the Massachusetts Institute of Technology, (TechnicalReport 854 Aug. 14, 1989) and the resulting U.S. Pat. No. 4,895,790, afabrication process for a binary diffractive optical element ormulti-level phase profile diffraction grating starts with a mathematicalphase description of a diffractive phase profile and results in afabricated multi-level diffractive surface. The first step is to takethe mathematical phase expression and generate from it a set of masksthat contain the phase profile information. The second step is totransfer the phase profile information from the masks into the surfaceof the element specified by the lens design.

The first step involved in fabricating the multi-level element is tomathematically describe the ideal diffractive phase profile that is tobe approximated in a multi-level fashion. The next step in thefabrication process is to create a set of lithographic masks which areproduced by standard pattern generators used in the integrated circuitindustry.

A substrate of the desired material is coated with a thin layer ofphotoresist. The lithographic mask is then placed in intimate contactwith the substrate and illuminated from above with an ultravioletexposure lamp. Alternately, pattern generators, either optical orelectron beam, can expose the thin layer of photoresist. The photoresistis developed, washing away the exposed resist and leaving the binarygrating pattern in the remaining photoresist. This photoresist will actas an etch stop.

The most reliable and accurate way to etch many optical materials is touse reactive ion etching. The process of reactive ion etchinganisotropically etches material at very repeatable rates. The desiredetch depth can be obtained very accurately. The anisotropic nature ofthe process assures a vertical etch, resulting in a true binary surfacerelief profile. Once the substrate has been reactively ion etched to thedesired depth, the remaining photoresist is stripped away, leaving abinary phase surface relief grating.

The process is repeated using a lithographic mask having half the periodof the first mask. The binary phase element is recoated with photoresistand exposed using the second lithographic mask which has half the periodof the first mask. After developing and washing away the exposedphotoresist, the substrate is reactively ion etched to a depth half thatof the first etch. Removal of the remaining photoresist results in a 4level approximation to the desired profile. The process is repeated athird and fourth time with lithographic masks having periods ofone-quarter and one-eighth that of the first mask, and etching thesubstrates to depths of one-quarter and one-eighth that of the firstetch. The successive etches result in elements having 8 and 16 phaselevels.

This process is repeated to produce a multilevel phase relief structurein the substrate. The result is a discrete, computer-generated structureapproximating the original idealized diffractive surface. For eachadditional mask used in the fabrication process, the number of discretephase levels is doubled, hence the name "binary" optical element or,more precisely, a binary diffractive optical element.

After only four processing iterations, a 16 phase level approximation tothe continuous case can be obtained. This mask and etch fabricationprocess can be carried out in parallel, producing many elementssimultaneously, in a cost-effective manner.

A 16 phase level structure achieves 99 percent diffraction efficiency.The residual 1 percent of the light is diffracted into higher orders andmanifests itself as scatter. In many optical systems, this is atolerable amount of scatter. The fabrication of the 16 phase levelstructure is relatively efficient due to the fact that only fourprocessing iterations are required to produce the element.

The photolithographic etch steps can be done in any order.Alternatively, the highest pitch, shallowest level is processed firstsince this level is more difficult to control if etched following deeperetches.

After the first etching step, the second and subsequent lithographicmasks have to be accurately aligned to the existing pattern on thesubstrate. Alignment is accomplished using another tool standard to theintegrated circuit industry, a mask aligner.

As noted, the photoresist on the substrate can be exposed with anelectron-beam pattern generator. The e-beam direct-write processeliminates masks and their corresponding alignment and exposureproblems. Binary optics have also been reproduced using epoxy casting,solgel casting, embossing, injection molding and holographicreproduction.

Binary optical elements have a number of advantages over conventionaloptics. Because they are computer-generated, these elements can performmore generalized wavefront shaping than conventional lenses or mirrors.Elements need only be mathematically defined: no reference surface isnecessary. Therefore, wildly asymmetric binary optics are able tocorrect aberrations in complex optical systems, and elements can be madewavelength-sensitive for special laser systems.

The diffractive optical elements are generally thinner, lighter and cancorrect for many types of aberrations and distortions. It is possible toapproximate a continuous phase profile with a stepwise profile ofdiscrete phase levels.

Optical scanning systems are used to scan a spot of light along apredetermined pattern such as a scan line on a photoreceptor. Opticalelements such as refractive lens or diffractive holograms or gratingscan be disposed circumferentially in annular sectors around a rotatingdisc to generate a scan line and function as an optical scanning system.Even binary diffractive optical elements can be used as the scanningelements in a rotating disc optical scanner as taught in copendingpatent applications Ser. Nos. 07/989,445 and 07/989,320), commonlyassigned with the present application and herein incorporated byreference.

One of the problems inherent in any optical scanning system ismonitoring and synchronizing the scanning point of light along the scanline and from scan line to scan line. This is typically done withphotodetectors located on a photoreceptor located at the start of scanand the end of scan. But these photodetectors do not monitor andsynchronize the entire continuous scan, only the ends.

An alternate system would remove a portion of the scanning beam itselfto provide the monitoring and synchronizing function.

It is an object of this invention to provide an optical synchronizationsystem for a rotating disc using binary diffractive optical elements.

It is another object of this invention to provide an opticalsynchronization system over the entire continuous scan line.

It is another object of this invention to provide an opticalsynchronization system which does not use the scanning beam itself.

SUMMARY OF THE INVENTION

In accordance with the present invention, an optical synchronizationsystem uses a stationary binary diffractive optical element and arotating disc with alternating binary diffractive optical elements andblocking elements to create a moire pattern for a synchronization signalfor a pixel clock. The stationary binary diffractive optical element andthe alternating binary diffractive optical elements and blockingelements will have the same multilevel phase relief structurediffraction pattern. The optical synchronization system using astationary binary diffractive optical element and a rotating disc withalternating binary diffractive optical elements and blocking elementscan either be reflective or transmissive.

Other objects and attainments together with a fuller understanding ofthe invention will become apparent and appreciated by referring to thefollowing description and claims taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of the binary diffractive opticalelement disc for an optical synchronization system formed according tothe present invention.

FIG. 2 is a schematic illustration of the annular sectors of alternatingbinary diffractive optical elements and blocking elements of the binarydiffractive optical element disc of FIG. 1 formed according to thepresent invention.

FIG. 3 is a schematic illustration of a reflective opticalsynchronization system using a rotating binary diffractive opticalelement disc formed according to the present invention.

FIG. 4 is a schematic illustration of an alternate embodiment of areflective optical synchronization system using a rotating binarydiffractive optical element disc formed according to the presentinvention.

FIG. 5 is a schematic illustration of a transmissive opticalsynchronization system using a rotating binary diffractive opticalelement disc formed according to the present invention.

FIG. 6 is a schematic illustration of an alternate embodiment of atransmissive optical synchronization system suing a rotating binarydiffractive optical element disc formed according to the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference is now made to FIG. 1, wherein there is illustrated arotatable transparent disc 10 which has a plurality of binarydiffractive optical elements 12 disposed circumferentially in annularsectors around the disc to generate an optical synchronization signal,specifically a moire pattern, for a pixel clock.

As shown in FIG. 2, the binary diffractive optical elements 12 alternatewith blocking elements 14 in the annular sectors. The multilevel phaserelief structure of the binary diffractive optical element will diffractany incident light while the stop or light absorbing patch of theblocking element will prevent the transmission of any incident light.The binary diffractive optical element 12 and the blocking element 14have the same width and height and cover the same surface area on theannular sector of the rotatable transparent disc 10.

The multilevel phase relief structure of each binary diffractive opticalelement 12 is the same so that any incident light beam will bediffracted exactly the same.

The rotatable disc 10 also has the scanning elements 16 such asholograms, diffraction gratings, binary diffractive optical elements andthe like disposed circumferentially in annular sectors around the discto generate a scan line. A light beam (not shown) will be diffracted orrefracted by the scanning element to form a scanning beam (not shown) toscan a scan line (not shown). A plurality of the plurality ofalternating binary diffractive optical elements and blocking elementswill correspond to a scanning element so that the opticalsynchronization system for the rotating disc is for an optical scanningsystem on the rotating disc.

If the scanning elements 16 on the rotating disc 10 are binarydiffractive optical elements as disclosed in copending patentapplications Ser. Nos. 07/989,445 and 07/989,320, then the alternatingbinary diffractive optical elements 12 of the optical synchronizationsystem can be fabricated at the same time and by the same method as theoptical scanning system.

The optical synchronization system 18 of FIG. 3, has a light source 20,such as a visible diode laser separate from the optical scanning systemlight source, which emits a coherent light beam 22 of a singlewavelength. The light beam 22 is collimated by collimating lens 24,which may be a molded glass aspherical collimating lens.

The collimated beam 26 is then incident upon the binary diffractiveoptical elements 12 and the blocking elements 14 of the rotating disc10. The light beam 26 will typically have a normal or perpendicularangle of incidence to the rotating binary diffractive optical andblocking element disc 10.

The disc 10 is rotatable about an axis of rotation 28. The rotating discis driven at a constant speed by a motor (not shown) coupled to therotor (also not shown) along the axis of rotation of the disc. Therotational movement of the disc 10 will cause the alternating binarydiffractive optical elements 12 and the blocking elements 14 to alignand misalign with the alternating binary diffractive optical elementsand the blocking elements of the stationary binary diffractive opticalelement causing the incident light beam to be interrupted repeatedly asthe disc rotates. The multilevel phase relief structure of the binarydiffractive optical element 12 will transmit and diffract any incidentlight 26 while the blocking element 14 will prevent the transmission ofany incident light 26.

Each of the phase levels in the multilevel phase relief structure of thebinary diffractive optical element is a diffractive grating with thepitch varying over the entire element. As the element rotates with thedisc, the multilevel phase relief structure of the binary diffractiveoptical element moves through an incident light beam and this beam isdeflected according to the local pitch, resulting in a diffracted beam30.

The diffracted light 30 from the binary diffractive optical elements 12of the rotating disc 10 is incident upon a stationary binary diffractiveoptical element 32. The stationary binary diffractive optical element 32will function as a diffraction grating reticule. The multilevel phaserelief structure of the stationary binary diffractive optical element 32is the same as that of the multilevel phase relief structure of thebinary diffractive optical elements 12 and the blocking elements 14 ofthe rotating disc 10.

The stationary binary diffractive optical element 32 therefore consistsof alternating binary diffractive optical elements 34 and the blockingelements 36. The multilevel phase relief structure of the binarydiffractive optical element 34 will diffract any incident light 30 whilethe stop or light absorbing patch of the blocking element 36 willprevent the transmission of any incident light 30. The light beam 38deflected by the binary diffractive optical elements 34 of thestationary binary diffractive optical element 32 is reflected from amirror 40 through the stationary binary diffractive optical element 32again but from the opposite side. The incident beam 38 will be blockedby blocking element 36 or diffracted by the binary diffractive opticalelements 34 forming diffracted beam 41.

The beam 41 from the stationary binary diffractive optical element isthen incident upon the binary diffractive optical elements 12 and theblocking elements 14 of the rotating disc 10 from the opposing side. Theincident beam 41 will be blocked by blocking element 14 or diffracted bythe binary diffractive optical elements 12 to form diffracted beam 42.

The beam 42 is then focussed by collimating lens 24 to a point 44 wherea photodetector 46 is positioned to receive the light. Collimating lens24 acts as a focussing lens since the light is incident upon theopposing surface. The focusing position 44 is different from theposition of the light source 20 since the beam has been diffracted andthus displaced spatially.

Light alternately being blocked and propagating through the rotatingbinary diffractive optical element and then through the stationarybinary diffractive optical element will create a moire pattern since themultilevel phase relief structure is the same for both elements.

This moire pattern of alternating light upon the photodetector willgenerate a series of synchronization signals for a pixel clock for anoptical scanning system elsewhere on the rotating disc 10.

The detected signal by the photodetector is at a submultiple of therequired pixel clock frequency. The light will be chopped as theidentical diffraction patterns on the rotating binary diffractiveoptical element alternately align with blocking and diffractive elementson the stationary binary diffractive optical element. If the shape andspacing of the identical multilevel phase relief structure diffractionpatterns are chosen appropriately, the electronic signal developed atthe photodetector can be the pixel clock.

It is not essential to the present invention that the optical path befrom the rotating binary diffractive optical element disc to thestationary binary diffractive optical element to the stationary binarydiffractive optical element again and to the rotating binary diffractiveoptical element disc again.

As shown in FIG. 4, the positions of the stationary binary diffractiveoptical element 48 and the rotating binary diffractive optical elementdisc 50 can be transposed and the optical synchronization system 52 willstill perform the same functions.

Thus, a light source 54 emits a coherent light beam 56 which iscollimated by collimating lens 58. The light is then incident upon thestationary binary diffractive optical element 48. The light will beblocked by the blocking elements of the stationary binary diffractiveoptical element 48 and diffracted by the binary diffractive opticalelements of the stationary binary diffractive optical element 48.

The light is then incident upon the rotating binary diffractive opticalelement disc 50. The light will be blocked by the blocking elements ofthe rotating binary diffractive optical element disc 50 and diffractedby the binary diffractive optical elements of the rotating binarydiffractive optical element disc 50. The light is then reflected back bythe mirror 60 to the rotating binary diffractive optical element disc50. The light will be blocked by the blocking elements of the rotatingbinary diffractive optical element disc 50 and diffracted by the binarydiffractive optical elements of the rotating binary diffractive opticalelement disc 50.

The light is then incident upon the stationary binary diffractiveoptical element 48. The light will be blocked by the blocking elementsof the stationary binary diffractive optical element 48 and diffractedby the binary diffractive optical elements of the stationary binarydiffractive optical element 48. The light is then focussed by thecollimating lens 58 upon a photodetector 62.

It is also not essential that the optical synchronization system be areflective system. As shown in FIG. 5, the optical synchronizationsystem 64 can be a transmissive system and still perform the samefunctions.

Thus, a light source 66 emits a coherent light beam 68 which iscollimated by collimating lens 70. The light is then incident upon thestationary binary diffractive optical element 72. The light will beblocked by the blocking elements of the stationary binary diffractiveoptical element 72 and diffracted by the binary diffractive opticalelements of the stationary binary diffractive optical element 72.

The light is then incident upon the rotating binary diffractive opticalelement disc 74. The light will be blocked by the blocking elements ofthe rotating binary diffractive optical element disc 74 and diffractedby the binary diffractive optical elements of the rotating binarydiffractive optical element disc 74.

The light is then focussed by a focussing lens 76 upon a photodetector80.

Once again, it is not essential in a transmissive opticalsynchronization system that the optical path be from the stationarybinary diffractive optical element to the rotating binary diffractiveoptical element disc.

As shown in FIG. 6, the positions of the rotating binary diffractiveoptical element disc 82 and the stationary binary diffractive opticalelement 84 can be transposed and the optical synchronization system 86will still perform the same functions.

Thus, a light source 88 emits a coherent light beam 90 which iscollimated by collimating lens 92. The light is then incident upon therotating binary diffractive optical element disc 82. The light will beblocked by the blocking elements of the rotating binary diffractiveoptical element disc 82 and diffracted by the binary diffractive opticalelements of the rotating binary diffractive optical element disc 82.

The light is then incident upon the stationary binary diffractiveoptical element 84. The light will be blocked by the blocking elementsof the stationary binary diffractive optical element 84 and diffractedby the binary diffractive optical elements of the stationary binarydiffractive optical element 84.

The light is then focussed by a focusing lens 94 upon a photodetector96.

Any spectral dispersion in any of the optical synchronization systemscan be compensated by coordinating the positive dispersion of the glasscollimating lens with the negative dispersion inherent in the stationaryand rotating binary diffractive optical elements.

The incident light beam typically has a normal or perpendicular angle ofincidence to the rotating binary diffractive optical element disc. Theincident beam can be at the Bragg angle of incidence to reduce loss asthe beam strikes the front surface of the rotating binary diffractiveoptical element disc. The incident beam can, however, have any angle ofincidence to the rotating binary diffractive optical element disc.

The rotating binary diffractive optical element includes a negativebinary diffractive lens element repeated in the sections of the annulus.The lens' effective focal length is a function of the radius, areasnearer the axis of rotation have shorter focal lengths. This variableeffective focal length, increasing as the radius of the binarydiffractive optical element on the disc increases, is compensated by thestationary binary diffractive optical element. The effect of the twobinary diffractive optical elements is to bring the diffracted beam to afocus.

The rotating binary diffractive optical element need not be a negativeoptical element but can instead, be a positive element. A positiveelement would work with the stationary binary diffractive opticalelement to achieve focussing of the incident beam.

A wedge effect can incorporated in the binary diffractive opticalelements in the one plane to insure that the diffracted beam can bephysically and optically separated from the zero order, nondiffractedbeam. Light would be diffracted by the wedge effect toward the center ofthe axis of rotation (or equivalently away from the center of the axisof rotation).

While the invention has been described in conjunction with specificembodiments, it is evident to those skilled in the art that manyalternatives, modifications and variations will be apparent in light ofthe foregoing description. Accordingly, the invention is intended toembrace all such alternatives, modifications and variations as fallwithin the spirit and scope of the appended claims.

What is claimed is:
 1. An optical synchronization system for a rotatingdisc comprising:a light source for emitting a coherent light beam, meansto collimate said coherent light beam, a first stationary binarydiffractive optical element to alternately block and diffract saidcollimated light beam, a plurality of alternating second binarydiffractive optical elements and blocking elements disposedcircumferentially on said rotating disc such that said diffracted lightbeam from said first stationary binary diffractive optical element isalternately blocked by said alternating blocking elements and diffractedby said alternating second binary diffractive optical elements as saidrotating disc rotates, reflecting means to reflect said diffracted lightbeam from said second binary diffractive optical elements on saidrotating disc, such that said reflected light beam is alternatelyblocked by said alternating blocking elements and diffracted by saidalternating second binary diffractive optical elements on said rotatingdisc as said rotating disc rotates, and said diffracted reflected lightbeam is alternately blocked and diffracted by said first stationarybinary diffractive optical element, and focussing means to focus saiddouble diffracted, reflected light beam onto a photodetector to generatea synchronization signal for rotation of said rotating disc.
 2. Theoptical synchronization system for a rotating disc of claim 1 whereinsaid first stationary binary diffractive optical element and saidplurality of alternating second binary diffractive optical elements andblocking elements have the same diffraction pattern.
 3. The opticalsynchronization system for a rotating disc of claim 1 wherein said meansto collimate said coherent light beam and said focussing means to focussaid double diffracted, reflected light beam are the same opticalelement.
 4. An optical synchronization system for a rotating disccomprising:a light source for emitting a coherent light beam, means tocollimate said coherent light beam, a plurality of alternating firstbinary diffractive optical elements and blocking elements disposedcircumferentially on said rotating disc such that said collimated lightbeam is alternately blocked by said alternating blocking elements anddiffracted by said alternating first binary diffractive optical elementsas said rotating disc rotates, a second stationary binary diffractiveoptical element to alternately block and diffract said diffracted lightbeam from said first binary diffractive optical elements, reflectingmeans to reflect said diffracted light beam from said second stationarybinary diffractive optical element, such that said reflected light beamis alternately blocked and diffracted by said second stationary binarydiffractive optical element, and said diffracted reflected light beam isalternately blocked by said alternating blocking elements and diffractedby said alternating first binary diffractive optical element on saidrotating disc as said rotating disc rotates, and focussing means tofocus said double diffracted, reflected light beam onto a photodetectorto generate a synchronization signal for rotation of said rotating disc.5. The optical synchronization system for a rotating disc of claim 4wherein said plurality of alternating first binary diffractive opticalelements and blocking elements and said second stationary binarydiffractive optical element have the same diffraction pattern.
 6. Theoptical synchronization system for a rotating disc of claim 4 whereinsaid means to collimate said coherent light beam and said focussingmeans to focus said double diffracted, reflected light beam are the sameoptical element.
 7. An optical synchronization system for a rotatingdisc comprising:a light source for emitting a coherent light beam, meansto collimate said coherent light beam, a first stationary binarydiffractive optical element to alternately block and diffract saidcollimated light beam, a plurality of alternating second binarydiffractive optical elements and blocking elements disposedcircumferentially on said rotating disc such that said diffracted lightbeam from said first stationary binary diffractive optical element isalternately blocked by said alternating blocking elements and diffractedby said alternating second binary diffractive optical elements as saidrotating disc rotates, and focussing means to focus said doublediffracted light beam from said plurality of alternating second binarydiffractive optical elements onto a photodetector to generate asynchronization signal for rotation of said rotating disc.
 8. Theoptical synchronization system for a rotating disc of claim 7 whereinsaid first stationary binary diffractive optical element and saidplurality of alternating second binary diffractive optical elements andblocking elements have the same diffraction pattern.
 9. An opticalsynchronization system for a rotating disc comprising:a light source foremitting a coherent light beam, means to collimate said coherent lightbeam, a plurality of alternating first binary diffractive opticalelements and blocking elements disposed circumferentially on saidrotating disc such that said collimated light beam is alternatelyblocked by said alternating blocking elements and diffracted by saidalternating first binary diffractive optical elements as said rotatingdisc rotates, a second stationary binary diffractive optical element toalternately block and diffract said diffracted light beam from saidfirst binary diffractive optical elements, and focussing means to focussaid double diffracted light beam from said second stationary binarydiffractive optical element onto a photodetector to generate asynchronization signal for rotation of said rotating disc.
 10. Theoptical synchronization system for a rotating disc of claim 9 whereinsaid plurality of alternating first binary diffractive optical elementsand blocking elements and said second stationary binary diffractiveoptical element have the same diffraction pattern.